(1) Field of the Invention
The present invention relates to a non-chromate conversion coating and method of treating a metal surface with same, and more particularly, to a xe2x80x9cdrop-in replacementxe2x80x9d, such as a titanate, for a chromate in a conventional conversion coating solution that otherwise would contain the chromate.
(2) Description of the Prior Art
It is known that solutions containing hexavalent chromium can be used to treat the surface of a metal, such as aluminum, to effectively keep the metal surface from rusting. However, although hexavalent chromium is an efficient rust-proofing agent, it is highly toxic and adversely affects the environment and human health. For this reason, many chromate-free chemical conversion coatings for metal surfaces have been proposed.
Thus, various non-chromate conversion coatings, such as the conversion coatings described in Tomlinson U.S. Pat. No. 5,759,244, the disclosure of which is incorporated by reference herein, have been disclosed which are designed to render a metal less reactive in a corrosive environment. Such non-reactive or less reactive metal surfaces produce a corrosion resistant outer layer on the base metal or its oxide thereby leaving the underlying metal protected from the environment. These coatings are applied in one or more stages and are subsequently rinsed with water to remove undesirable contaminants.
Chromate-free conversion coatings are therefore generally based on chemical mixtures that react with a metal substrate surface to form a protective layer. Many of these conversion coatings are based on Group IV-B metals such as titanium, zirconium and hafnium. For example, U.S. Pat. No. 5,743,971 to Inoue et al discloses a rust proof film-forming composition for treating a metal surface comprising an oxidated substance, a silicate and/or silicone dioxide and at least one member selected from the group consisting of metal cations of titanium, zirconium, cerium, strontium, vanadium, tungsten, and molybdenum. A metal substrate is provided a rust proof film by immersing it in the foregoing liquid rust proof film-forming composition. Similarly, U.S. Pat. No. 5,855,695 to McMillen et al discloses a non-chrome passivating composition employed as a post-rinse for enhancing the corrosion resistance of phosphated metal substrates. The composition comprises the reaction product of an epoxy-functional material containing at least two epoxy groups and an alkanolamine, or a mixture of alkanolamines. The non-chrome passivating composition further comprises a Group IV-B metal ion, or a mixture of Group IV-B metal ions. Moreover, U.S. Pat. No. 5,897,716 to Reghi et al discloses a chemically and thermally stable chromate-free aqueous liquid treatment for metals for imparting corrosion resistance thereto. The chromate-free aqueous liquid comprises components selected from the group consisting of H2TiF6, H2ZrF6, H2HfF6, H2SiF6, H2GeF6, H2SnF6, HBF4 and mixtures thereof.
The shortcoming of conventional non-chromate conversion coatings, such as those described above, is that they cannot be integrated into and employed in place of chromates in current metal treatment coatings which employ chromates. As such, conventional non-chromate conversion coatings are usually sufficiently different from previously employed chromate-containing conversion coatings that significant changes are required to be made in the metal treating process and in the production of the conversion coating itself. These changes can amount to substantial expenditures and usually require additional approvals from the Department of the Navy or a regulatory agency of the United States Government. Thus, there is a need for a xe2x80x9cdrop-in replacementxe2x80x9d that can be employed in place of chromate compounds, such as sodium dichromate, now used in conventional chromate conversion coatings. xe2x80x9cDrop-in replacementxe2x80x9d refers to a compound that can be employed in a conventional conversion coating in lieu or in place of a chromate without requiring any or substantial changes in the make-up of the conversion coating or its substituents.
It is a primary object of the present invention to provide a non-chromate conversion coating for treating metals which contains a titanate in place of a chromate.
It is a further primary object of the invention to provide a xe2x80x9cdrop-in replacementxe2x80x9d for a chromate that can be employed in a conversion coating which otherwise would employ a chromate.
It is a further primary object of the invention to provide a method of rust-proofing a metal substrate by applying a non-chromate titanate conversion coating thereto.
Another object of the invention is to provide a one-stage method of rust proofing a metal substrate by applying a non-chromate titanate conversion coating thereto including sodium metatinate and/or potassium titanate.
Another object of the invention is to provide a non-chromate conversion coating that excludes therein organic additives, structural component additives or chelating agents.
The objects of the invention are accomplished by providing a highly effective, non-chromate conversion coating which includes a titanate, such as sodium metatitanate or potassium titanate, in lieu of a chromate in a typical conversion coating that otherwise would contain a chromate.
The present invention is developed on the basis of findings that an excellent rust proof film can be obtained by immersing a metal substrate in an aqueous solution which includes sodium dichromate, sodium fluoride, potassium ferricyanide and nitric acid in an amount to provide a pH of 1.2 to 2.2. It is believed that the chromate provides corrosion protection by way of a cathodic reaction, specifically, the reduction of oxygen in the presence of water:
O2+2H2O+4exe2x88x924OHxe2x88x92xe2x80x83xe2x80x83(1)
This cathodic reaction is similar for many systems, and by changing the oxygen concentration in the solution, reveals the cathodic behavior of the chromate. Moreover, when the reduction of oxygen is the rate controlling reaction and chromates are present, other metals and lower oxygen levels show similar behavior, that is a lower or decreased limiting current density.
Test results show that a metal tested without a conversion coating has a high limiting cathodic density. For example, untreated Al2024T3 has a limiting cathodic current density of 10-20 A/cm2, however, when a chromate conversion coating is applied, the cathodic limiting current density is lowered to 3-7 A/cm2. However, since personal exposure limits (PEL) for chromates is 0.1 mg/m3 (milligram per cubic meter), chromate containing conversion coatings are not practical for use. Thus, a xe2x80x9cdrop-in replacementxe2x80x9d for the chromate in the chromate-containing conversion coating is highly desired.
Sodium metatitanate and potassium titanate have been found to be well suited as xe2x80x9cdrop-in replacementsxe2x80x9d for chromates in conversion coatings which, in addition to sodium dichromate, contain sodium fluoride, potassium ferricyanide and nitric acid. For example, test results show that a conversion coating which includes a xe2x80x9cdrop-in replacementxe2x80x9d according to this invention in place of a chromate produces a metal surface having a cathodic limiting current density of 0.5 to 1 A/cm3. Furthermore, the PEL for such a conversion coating is 15 mg/m3. Thus, the present invention provides a highly effective, non-toxic conversion coating which otherwise would include toxic chromate compounds, such as sodium dichromate.